Miniaturized photoacoustic spectrometer

ABSTRACT

A miniaturized photoacoustic spectrometer made from a series of stacked substrates. An infrared source is formed in a first substrate. A filter is formed in a second substrate. A micro-trough is machined in a third substrate and a microphone is formed in a fourth substrate. A fifth substrate has a metallic deposit for reflecting light emitted by the infrared source. Resin sealing is provided between the substrates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoacoustic spectrometry device,used for example for analysing gases. It more particularly relates to aminiaturized photoacoustic spectrometer. This device is implemented fromstacks of elementary components, capable of being implemented bysubstrate etching, metallic deposition and substrate assemblytechniques, of the type used in microelectronics.

2. Discussion of the Background

The principle of photoacoustic spectroscopy for analysing gases has beendiscussed in the article by J. Christensen entitled “The Brüel KjaerPhotoacoustic Transducer System and its Physical Properties”. The devicedescribed in this document has:

an infrared hot source,

a mechanical “chopper”, which modulates the intensity of the source,

an interferential optical filter,

a cylindrical trough,

two matched microphones, the sum of the signals originating from thesemicrophones making it possible to double the photoacoustic signal andnullify the noise due to external vibrations.

In this device, the source is at a distance from the trough in order toavoid any heating of the gas. To that end, the use of an ellipsoidalmirror coupled to the source makes it possible to achieve suitablecollimation of the light beam.

The document WO-96/24831 describes a photoacoustic detector having achamber for receiving a gas to be measured, an infrared light beamcapable of passing through this chamber, and a pressure sensor capableof measuring the pressure variations in the chamber, which are inducedby an infrared beam. The chamber is formed by the assembly of twosemiconductor elements, for example silicon or quartz elements,implemented in planar technology. The pressure variations are detectedby means of a membrane.

Implementation of a photoacoustic spectrometer, of miniature size,allowing integration of all the elements (radiation source, filter,trough, microphone) in a compact manner, is not known.

SUMMARY OF THE INVENTION

An object of the invention is a photoacoustic spectrometer having aninfrared source which can be modulated electrically, an interferentialoptical filter, a microphone and a micro-trough, each of these elementsbeing integrated on a semiconductor substrate or on at least one, orwith the help of at least one semiconductor substrate, for example madeof silicon.

An object of the invention is therefore a photoacousticmicrospectrometer, obtained by assembly or sealing of four elementsintegrated on a semiconductor: an infrared source which can be modulatedelectrically, an interferential optical filter, a micro-trough, and amicrophone.

Each of the elements composing the spectrometer according to theinvention can be integrated on, or implemented with the help of, one ortwo semiconductor substrates.

According to a first particular embodiment, an object of the inventionis a photoacoustic spectrometer having:

an infrared source implemented in a first semiconductor substrate,

a filter implemented with the help of a second semiconductor substrate,

a micro-trough formed in a third semiconductor substrate,

a microphone implemented with the help of the third semiconductorsubstrate and a fourth semiconductor substrate.

According to a second particular embodiment, an object of the inventionis a photoacoustic spectrometer, having:

an infrared source implemented in a first semiconductor substrate,

a Fabry-Pérot interferential filter formed with the help of a second anda third semiconductor substrates,

a micro-trough Implemented partially in the third semiconductorsubstrate and partially in a fourth semiconductor substrate,

a microphone implemented with the help of the fourth and a fifthsemiconductor substrates.

According to a third particular embodiment, an object of the inventionis a photoacoustic spectrometer, having:

an infrared source, implemented in a first semiconductor substrate,

a Fabry-Pérot interferential filter formed with the help of the firstand a second semiconductor substrates,

a micro-trough implemented in the second semiconductor substrate,

a microphone implemented on the surface of a third semiconductorsubstrate.

According to a fourth particular embodiment, an object of the inventionis a photoacoustic spectrometer, having:

an infrared source implemented in a first semiconductor substrate,

a Fabry-Pérot interferential filter implemented with the help of thefirst and a second semiconductor substrates,

a microphone and a micro-trough, implemented in the second semiconductorsubstrate.

In the device according to the invention, the mechanical chopping of thebeam can be replaced by direct electrical modulation of the injectioncurrent in the infrared source.

The source can have a metallic grid, or a metallic filament, supportedby a membrane above a cavity etched in a semiconductor substrate. Thisgrid, or this filament, is for example made of silicon nitride, orplatinum, or tantalum, or titanium, or tungsten, or molybdenum, orchromium, or nickel, or one of their alloys, or TiN.

Preferably, the source is placed in a cavity. Putting this cavity undervacuum moreover makes it possible to avoid heating problems due to thegaseous medium, which can be critical in a miniature device.

The interferential filter can be a filtering substrate.

This can also be a Fabry-Pérot tunable filter.

It can then have a first, fixed, mirror and a second, movable, mirror,these mirrors delimiting, at rest, a resonant cavity of length d, firstand second control electrodes being associated respectively with thesefirst and second mirrors, the application of an electrical voltagebetween the control electrodes allowing implementation of a displacementof the movable mirror with respect to the fixed mirror, and thereforemodifying the length d of the resonant cavity.

According to another embodiment, the Fabry-Pérot tunable filter has:

a first mirror, with which a floating electrode is associated,

a second mirror, with which a first and a second control electrode areassociated, one out of the first and second mirrors being fixed whilethe other is movable,

a resonant cavity, of length d, delimited by the first and secondmirrors, the application of an electrical voltage between the twocontrol electrodes bringing about a displacement of the movable mirrorwith respect to the fixed mirror and therefore modifying the length d ofthe resonant cavity.

In this embodiment, the electrode associated with one of the mirrors isa floating electrode, and no contact connection is to be implemented onthe side of this mirror. There is therefore, in this system, only asingle level of contact to be made, corresponding to the controlelectrodes. The device is therefore easier to implement, since a contactconnection on both levels of mirror is difficult and requires a localstack of highly doped layers.

The fixed and movable mirrors can be implemented by stacking ofmultilayers at λ/4, on the surface of semiconductor substrates.

The movable mirror can be implemented by a membrane situated above acavity implemented in a semiconductor substrate.

The floating electrode and the corresponding mirror can be implementedon the surface of one of the semiconductor substrates.

The control electrodes and the corresponding mirror can be implementedon the surface of another of the semiconductor substrates.

For example, the movable mirror can be composed of a membrane etched inthe second semiconductor substrate.

As for the control electrodes, they can be formed on either side of areflective area of the mirror with which they are associated. In otherwords, this mirror has a reflective central area, and lateral areas onwhich the control electrodes are formed.

This reflective central area can have a circular form. This circularform, delimited by the control electrodes, allows, if the correspondingmirror is movable, a perfectly plane displacement of the movablereflective area, since the electrostatic attraction takes place only atthe periphery of this area, which makes it possible to have adiaphragmed filter output.

The control electrodes can be implemented in a metallic deposit.Furthermore, electrical contacts can be made directly on the controlelectrodes, on the surface of the substrate on which they are formed.

Preferably, the control electrodes also form an input diaphragm of themicro-trough.

According to another aspect, one of the walls of the micro-trough isconstituted by the microphone membrane.

According to yet another aspect of the invention, the microphone canhave a membrane, a first electrode associated with, or formed on, thismembrane, and a second electrode, the excess pressures in themicro-trough being detected by variation in the capacitance of the airgap defined by the first and second electrodes.

According to yet another aspect, the microphone has a membrane end afirst electrode associated with this membrane, both implemented on asemiconductor substrate, and a second electrode implemented on anothersemiconductor substrate.

When one of the walls of the micro-trough is constituted by themicrophone membrane, the said membrane can be implemented in a copedsemiconductor material, the microphone also having a counter electrode.The latter makes it possible to detect the vibrations of the membrane bymeasuring the variation in the capacitance formed by the membrane ofdoped semiconductor material and the counter electrode.

The microphone membrane can then be situated on the surface of asubstrate, etched under the membrane.

When the tunable filter has a membrane, the latter can be used as both afilter membrane and a microphone membrane. In this case, control of thefilter and detection of the pressure variations are carried out with thehelp of one and the same system. The same electrodes as those whichcontrol the filter can then be used to measure the excess pressurecreated in the micro-trough, that is to say that filter and microphoneare as it were combined, and reduced to a single common membrane. Inthis case, and in order not to create any parasitic photoacoustic signalin the cavity due to absorption of the gas to be measured or of anothergas present at the same time, the cavity is preferably placed under aneutral atmosphere, for example under argon. In principle, the excesspressures created in the trough are sufficiently small for the tuning ofthe filter to the wavelength not to be lost: however, and in order tolimit any detuning, means for automatic control position-wise of themembrane can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In any case, the characteristics and advantages of the invention willemerge more clearly in the light of the description which follows. Thisdescription relates to the example embodiments, given by way ofexplanation and being non-limitative, referring to accompanying drawingsin which:

FIG. 1 depicts a first embodiment of a photoacoustic spectrometeraccording to the invention.

FIG. 2 depicts a second embodiment of a photoacoustic spectrometeraccording to the invention.

FIG. 3 depicts a third embodiment of a photoacoustic spectrometeraccording to the invention.

FIG. 4 depicts a fourth embodiment of a photoacoustic spectrometeraccording to the invention.

FIGS. 5A to 5K depict steps of implementing a device according to theinvention.

FIGS. 6A to 6J depict steps of implementing another device according tothe invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to a first embodiment, a device according to the invention 1is composed of an infrared source under vacuum (wafers, or substrates, 2and 4 of a semiconductor material), a standard optical filter (wafer, orsubstrate, 6 of a semiconductor material) , a micro-trough (wafer, orsubstrate, 8 of a semiconductor material) 12 and a capacitive microphone14 (wafers, or substrates, 8 and 10 of a semiconductor material) , thatis in total five substrates, four of which are machined.

The wafers 2 and 4 (for example made of standard Si) form the infraredsource enclosed in a cavity under vacuum 16, 20. This is for example afull sheet TiN layer with small holes made in it, coated with silica,and carried by a silicon nitride membrane 18. The membrane is freed fromthe front face of the wafer 4 by etching of a sacrificial layer (resinor tungsten (W)), thus leaving a cavity 20 in the substrate. The coverformed by the wafer 2, and in which a cavity 16 is etched, makes itpossible to place the source under vacuum when the sealing 22 is carriedout. A metallic deposit 24 on the bottom of the cavity 6 forms areflecting means and makes it possible to recover almost all the lightemitted by the source. Two other small cavities 26, 28, at both ends ofthe cover, facilitate contact connection in the contact connection areas30, 32 as well as cutting.

The source is for example modulated around 20 Hz.

The sealing 22 is for example resin sealing, under vacuum.

The wafer 6 constitutes the filter. It is inserted into the system whenthe final sealing is carried out.

The trough 12 is machined in the wafer 8 (this is for example an SOIwafer, with an SiO₂ layer of thickness 0.7 μm, and a doped Si layer, 0.3μm thick; standard SOI can also be taken, for example with a 0.4 μm SiO₂layer and an Si layer, 0.2 μm thick) at the time of etching a membrane34 of the microphone. A row of silica pads 36 makes it possible tocontrol the thickness of the air gap 14 between the wafers 8 and 10.Finally, metallic deposits 38, 39 on the two opposite faces constitutetwo measuring electrodes, a floating electrode being formed by themembrane 39. The sealings 40, 42, 44 can be resin sealings, whichprovide at least one vent for equalizing the external and internalpressures. The diameter of the vent is calculated to have a low cut-offfrequency, typically 1 Hz.

To give an order of magnitude for the different elements, the membrane18 of the source can have a surface area of 2×2 mm², the membrane 34 ofthe microphone having the same size, the trough has a width of around 2mm and the vent a diameter of around 10 μm.

The stack of FIG. 2 repeats that of FIG. 1 while integrating a tunablefilter. Numerical references identical to those of FIG. 1 designateidentical or corresponding elements therein. The stack is obtained bysealing six semiconductor substrates 2, 8, 10, 50, 52, 54, three ofwhich are vacuum-tight.

The wafers 2 (which serves as a cover) and 50 form the source whosemembrane 18 is freed by the rear face of the wafer 50. The wafers 2 and50 are for example standard silicon substrates.

The wafer 52 is for example an SOI wafer (for example: an SiO₂ layer ofthickness 0.7 μm and an Si layer of thickness 0.3 μm). It is used amongother things for the rear face freeing of a membrane 56 of the tunableFabry-Pérot.

A row of silica pads 58 of thickness λ/2, λ being the workingwavelength, makes it possible to control the width of a resonant cavity60 (the Fabry-Pérot cavity). An annular metallic deposit 62 on the frontface of the wafer 54 forms the control electrode of the filter, and alsoprovides a diaphragm function at the input of the micro-trough 67. Themembrane 56 of the wafer 52 forms a floating electrode. The faces 56, 57of the substrates 52, 54 are made reflecting by deposition of dielectricmultilayers (at λ/4) . This stack has the advantage of leaving theoptical path free between the source and the filter.

The resin sealings 22 and 64 are preferably implemented underatmosphere. The sealing 66 makes it possible to place thesource/Fabry-Pérot assembly under vacuum. Placing the filter undervacuum gives a good mechanical behaviour to the membrane 56.

A volume 67 is etched on the rear face of the wafer 54 (which is of SOItype (for example: SiO₂ of thickness 0.7 μm and Si of thickness 0.3μm)), for example over one half of the substrate, depth-wise, so thatits aperture is identical to that of the diaphragmed output of thefilter, in order to minimize the dead volumes.

The microphone 14 is implemented in the wafers 8 and 10: the rear faceetching of a volume 12 in the wafer 8 (of SO type (for example: SiO₂ ofthickness 0.7 μm and Si of thickness 0.3 μm)), allows the freeing of adetection membrane 34 of doped silicon, so that the trough has a depthof around a substrate and a half (volume 67+volume 12). A metallicdeposit 38 on the front face of he counter-wafer 10 (standardsemiconductor or standard substrate) constitutes the counter electrodeof the microphone, a floating electrode being formed by the membrane 34.The thickness of the air gap 14 is controlled by the same principle ofsilica pads 36 as that used for the filter 58.

The resin sealings 68, 44 of the wafers 54, 8 and 10 can provide a ventfor equalizing the internal and external pressures.

Lateral cavities 26, 28 in the wafers 2, 52 and 8 make it possible todefine the cutting path for freeing the contacts.

A simplified stack is depicted in FIG. 3. Numerical references identicalto those of FIGS. 1 or 2 designate identical or corresponding elementstherein. The wafers 2 and 4 form the source under vacuum carried by asilicon nitride membrane 18 freed by the front face of the wafer 4. Thesealing 22 is a resin sealing, under vacuum.

The wafers 4 and 70 (of SOI type: for example: SiO₂ of thickness 0.7 μmand Si of thickness 0.3 μm), simultaneously form the filter and thetrough 72. Silica pads 74 define the air cavity between two dielectricmirrors 76 and 78. A metallic deposit 80 on the membrane 76 forms thecontrol electrode and can also provide the diaphragm function. Thefloating electrode is formed by the doped silicon layer at the surfaceof the mirror 78. The cavities 26, 28 are etched on the rear face of thewafers 2 and 4 and 70 in order to clear the contacts 30, 32 and 80, 81,84 and 86 at the time of cutting.

The microphone is implemented in the wafer 72 (of SOI type: for exampleof standard SOI, with 0.4 μm of SiO₂ and 0.2 μm of Si; thicker layerscan also be used) by front face freeing no a doped silicon membrane 82.The measuring capacitance is formed by the membrane 82 and a counterelectrode (the counter electrode is formed by the surface 85 of thesubstrate 72 cleared after etching of the SiO₂ layer) by virtue ofcontacts 84, 86. The etched rear volume 88 eliminates fluid compressionproblems at the time of deflection of the membrane.

The resin sealings 90, 92 of the wafers 4 and 70 close the micro-roughon the microphone. A vent for equalizing the internal and externalpressures can be provided.

Finally, a simpler solution, illustrated in FIG. 4, consists of makingthe filter and microphone membranes common. This is a stack similar tothe previous one (FIG. 3), except for the wafers 4 and 70 (of SOI type:for example SiO₂ of thickness 0.7 μm and Si of thickness 0.3 μm) whichsimultaneously form the filter and the microphone with a common membrane76, and the trough 72. The metallic deposit 80 on the membrane 76 isused as both a control and a measuring electrode as well as, possibly, adiaphragm. The floating electrode is formed by the doped silicon layerat the surface of the mirror 78. The sealing 90 is implemented under aneutral atmosphere (Argon for example).

The last resin sealing 92 of the wafers 70 and 100 closes themicro-trough and can provide a vent for equalizing the internal andexternal pressures.

In the various embodiments disclosed above, the source has beendescribed by reference to a full sheet TiN layer with holes made in itand coated with silica. Other types of source can be implemented, withinthe context of a device according to the invention. In particular, thesource can be composed of a metallic grid or filament, supported by amembrane, above a cavity etched in a semiconductor substrate. The grid,or the filament, can for example be made of silicon nitride, orplatinum, or tantalum, or titanium, or tungsten, or molybdenum, orchromium, or nickel, or one of their alloys.

The source can for example be a miniaturized infrared radiation sourceas described in the document FR-96 11866 (dated Sep. 30, 1996). Such asource has at least one self-supported microfilament, comprising ametallic material, intended to emit infrared radiation under the actionof an electrical current passing through it. The microfilament can havethe form of a film of sufficiently small thickness to have a low thermalinertia, compatible with the emission periods of the infrared radiation.The microfilament can be composed of a metallic strip covered with atleast one thin layer of a material improving the emissivity of themicrofilament in at least part of the infrared spectrum.

The metallic material can be chosen from among the list of materialsalready given above. The material improving the emissivity of themicrofilament is preferably chosen from among the nitrides (for exampleSi₃N₄) , silicides (for example SiC, SiMo) , oxides (for example SiO₂,Al₂O₃) or borides.

In every case, the radiation source generates infrared radiation whichpasses through the interferential filter and reaches the trough, whereits absorption takes place. The microphone is disposed so as to detect apressure variation in the trough.

In the embodiments described above in relation to FIGS. 3 and 4, thefilter used is an electrostatically controlled and tunable Fabry-Pérotinterferential filter.

This structure has a first, fixed, mirror 78, with which a floatingelectrode 79 is associated. The latter can be constituted by formationof a doped layer in the substrate 4, forming an integral part of themirror: this is the external layer of the dielectric mirror. Facing thefixed mirror 79 there is a second mirror 76, movable along the axis XX′of the spectrometer. With this second mirror there are associated twocontrol electrodes designated by the reference 80. The said electrodesare implemented for example by metallization of part of a reflectingmembrane forming the second mirror 76.

The two mirrors are kept at a distance d with respect to one another.This distance is in fact the length of the resonant cavity delimited bythe mirrors 76, 78.

The two mirrors are kept at a distance d, in FIGS. 3 and 4, with thehelp of pads 74. This can also be achieved with the help of braces orcross-pieces.

The application of an electrical voltage, with the help of means notdepicted in the figures, between the control electrodes 80, brings abouta displacement of the movable mirror 76 with respect to the fixedmirror, along the axis XX′, and therefore modifies the length of theresonant cavity.

The length d of the Fabry-Pérot cavity satisfies the relationship:

2nd=mλ  (1)

where d is the length which separates the reflective surfaces, m is aninteger number, n is the refractive index of the medium situated betweenthe two mirrors and λ is the wavelength. Any variation in d thereforebrings about a variation in the central wavelength of the passband ofthe interferometer.

The cavity thus formed defines, from the electrical point of view, acapacitance C₁ between the control electrodes 80 and the floatingelectrode 79. In fact, the application of an electrical voltage betweenthe control electrodes modifies, through the capacitance C₁ of the airgap, the potential of the floating electrode 79 and, thus, the movablemirror 76 is attracted in the direction of the fixed mirror.

The floating electrode can be implemented on the substrate 70 (it isthen associated with, or implemented on, the movable mirror 76), thecontrol electrodes being associated with the fixed mirror.

A control electrode associated with each mirror (fixed and movable) canalso be implemented. Nevertheless, the implementation of a system with afloating electrode makes it possible to avoid one level of contactconnections, which simplifies the device and its method ofimplementation.

In FIG. 2, it is the membrane 56 which carries on its surface thefloating electrode (doped Si layer).

A method of implementing a device of the type already described above inrelation to FIG. 2 will be described in relation to FIGS. 5A to 5K.

In a first step (FIG. 5A) a semiconductor substrate 2 (preferably ofsilicon) is etched, and a metallic deposit 17 is implemented at thebottom of one of the chambers obtained by etching.

Next (FIG. 5B) an Si₃N₄—SiO₂—TiN deposit 19 is implemented on the frontface of a semiconductor substrate 50.

This deposit 19 is etched (FIG. 5C). Then, freeing of the membrane isimplemented by the rear face of the substrate, by etching the latter(FIG. 5D).

Next (FIG. 5E) an SiO₂ deposit is implemented on the rear face of an“SOI” type wafer 52, and pads 58, 60 are etched in this layer. Thesubstrate 52 is next etched so as to implement the membrane 56.

On another “SOI” type substrate 54, a metallic deposit is implemented onthe front face, which is etched in order to obtain the controlelectrodes 62 (FIG. 5G). The substrate 54 is next etched (FIG. 5H) so asto clear the micro-trough 66.

A fourth wafer 8, also of “SOI” type, is etched so as to implement themembrane 34 on the front face (FIG. 5I). A metallic deposit 35 isimplemented on the front face of the substrate 8 (FIG. 5J).

On an “SOI” type substrate 10, a metallic deposit, for example of gold,is implemented, which is next etched so as to implement the microphoneelectrodes 38 (FIG. 5K).

Silica pads next make it possible to adjust the different wafers withrespect to one another. Resin sealings 22, 64, 66, 68, 44 next make itpossible to obtain the stack illustrated in FIG. 2.

A method of implementing the devices described above in relation toFIGS. 3 and 4 will now be described in relation to FIGS. 6A to 6J.

First of all, in a first step (FIG. 6A) a first wager 2 of asemiconductor material (preferably silicon) is etched, so as to clearcavities therein. At the bottom of one of the cavities, a metallicdeposit 17 is implemented.

Then (FIG. 6E) an Si₃N₄—SiO₂—TiN deposit 19 is implemented on the rearface of an “SOI” type substrate 4.

Holes are next etched in this deposit (FIG. 6C).

Then (FIG. 6D) an SiO₂ deposit is implemented and etched so as to opencontact supports thereon: a gold deposit on these contact supportsallows implementation of the contacts 32 (FIG. 6D).

An SiO₂ deposit is next implemented on the front face, then etched so asto clear the pads 22 (FIG. 6E).

The membrane 18 is next freed by etching (FIG. 6F), in the area situatedunder the holes etched in the layer 18. Lateral cavities can also becleared to facilitate contacts to be made on the substrate opposite.

Next (FIG. 6G) a metallic deposit 80 is implemented, then etched, on thefront face of a wafer 70, of the “SOI” type. This wafer is next etchedon the rear face (FIG. 6H) so as to clear the membrane 76 and form themicro-trough 72.

The device of FIG. 4 can then be implemented, by assembling the elementsdescribed above with a fourth wafer 100 of semiconductor material. Theassembly is implemented in the same manner as described above (use ofpads and sealing by resin seals).

For the embodiment of FIG. 3, the method continues with the stepsillustrated in FIGS. 6I and 6J.

An “SOI” type wafer 72 is etched so as to clear the rear volume 88 ofthe microphone, behind the membrane of the latter.

Then (FIG. 6J) a hole 89 freeing the membrane is etched, and the latteris freed by the front face of the substrate 72. Lateral etching and ametallic deposit allows implementation of the contacts 84, 86 (see FIG.3). The assembly of this substrate with the preceding substrates isimplemented in the manner already described above.

A method for implementing a device as described above in relation toFIG. 1 includes the steps for implementing and machining the wafers 2, 4of the device of FIG. 3 (but without the implementation of the mirror 78and the electrode 79) and steps for implementing the wafers 8, 10 of thedevice of FIG. 2 (implementation of the micro-trough and themicrophone). It therefore suffices to select the necessary method stepsfrom among those described above in relation to FIGS. 5A-5K and 6A-6J.

For all the embodiments described above, the substrates selected arepreferably silicon substrates. Thus, the machined wafers havethicknesses of the order of a few hundred micrometres (between 100 μmand 1 mm, for example 450 or 500 μm). The superposition of 4, 5 or 6machined wafers with the corresponding elements, in accordance with theinvention, therefore leads to a device having a total thickness between500 μm and 2 to 3 mm. The micro-trough 12 (FIGS. 1, 2), 72 (FIGS. 3, 4)typically has a section of 2 mm×2 mm, while the membrane 34 (FIGS. 1,2), 76 (FIGS. 3, 4) has a thickness typically between 0.1 μm and a fewmicrometres (up to 10 μm, for example: 5 μm).

Furthermore, a substrate bonding technique has been described above;pads are used to control the air thickness between two substrates, whilethe sealings are carried out with the help of resin at high temperature(200° C.). Other substrate assembly techniques, derived frommicroelectronics, can also be used. For example, the so-called “AnodicBonding” technique (sealing of silicon on glass and under an electricfield at 400° C.) allows implementation of a device according to theinvention.

Use can also be made of the so-called SDB (“silicon direct bonding”)technique (sealing of oxide on oxide at high temperature (>800° C.)) orthe eutectic soldering technique described in “sensors and actuators”,A45 (p. 227-236).

What is claimed is:
 1. A photoacoustic spectrometer having: an infraredsource implemented in a first semiconductor substrate, a filterimplemented with the help of a second semiconductor substrate, amicro-trough formed in a third semiconductor substrate, a microphoneimplemented with the help of the third semiconductor substrate and afourth semiconductor substrate.
 2. A photoacoustic spectrometer, having:an infrared source implemented in a first semiconductor substrate, aFabry-Pérot interferential filter formed with the help of a second and athird semiconductor substrate, a micro-trough implemented partially inthe third semiconductor substrate and partially in a fourthsemiconductor substrate, a microphone implemented with the help of thefourth and a fifth semiconductor substrate.
 3. A photoacousticspectrometer, having: an infrared source, implemented in a firstsemiconductor substrate, a Fabry-Pérot interferential filter formed withthe help of the first and a second semiconductor substrate, amicro-trough implemented in the second semiconductor substrate, amicrophone implemented on the surface of a third semiconductorsubstrate.
 4. A photoacoustic spectrometer, having: an infrared sourceimplemented in a first semiconductor substrate, a Fabry-Pérotinterferential filter implemented with the help of the first and asecond semiconductor substrates, a microphone and a micro-trough,implemented in the second semiconductor substrate.
 5. A spectrometeraccording to claim 1, the infrared source being placed in a cavity undervacuum.
 6. A spectrometer according to claims 1, an etched substrateclosing the infrared source, a reflecting element being disposed in acavity etched in this substrate.
 7. A spectrometer according to claim 1,the interferential filter being a filtering substrate.
 8. A spectrometeraccording to claim 1, one of the walls of the micro-trough beingconstituted by the microphone membrane.
 9. A spectrometer according toclaim 1, the microphone having a membrane, a first electrode associatedwith this membrane, and a second electrode, the excess pressures n themicro-trough being detected by variation in the capacitance of the airgap between the first and second electrodes.
 10. A method ofimplementing a photoacoustic spectrometer, in particular according toclaim 1, including: the implementation of an infrared source in a firstsemiconductor substrate, the implementation of an interferential filterwith the help of a second semiconductor substrate, the implementation ofa micro-trough in a third semiconductor substrate, the implementation ofa microphone with the help of the third and a fourth semiconductorsubstrates, the assembly of the infrared source, the filter, themicro-trough and the microphone by assembly of the substrates.
 11. Amethod of implementing a photoacoustic spectrometer, in particularaccording to claim 2, including: the implementation of an infraredsource in a first semiconductor substrate, the implementation of atunable interferential filter with the help of a second and a thirdsemiconductor substrates, the implementation of a micro-trough,partially in the third semiconductor substrate and in a fourthsemiconductor substrate, the implementation of a microphone with thehelp of the fourth and a fifth semiconductor substrates.
 12. A method ofimplementing a photoacoustic spectrometer, in particular according toclaim 3, including: the implementation of an infrared source in a firstsemiconductor substrate, the implementation of a filter with the help ofthe first and a second semiconductor substrates, the implementation of amicro-trough, in the second semiconductor substrate, the implementationof a microphone, on the surface of a third semiconductor substrates, theassembly of the infrared source, the filter, the micro-trough and themicrophone by assembly of the substrates.
 13. A method of implementing aphotoacoustic spectrometer, in particular according to claim 4,including: the implementation of an infrared source in a firstsemiconductor substrate, the implementation of an interferential filterand a microphone with the help of the first and a second semiconductorsubstrates, the implementation of a micro-trough in the secondsemiconductor substrate, the assembly of the infrared source, thefilter, and the microphone and the micro-trough by assembly of the firstand second substrates.
 14. A method according to claim 12, themicrophone being implemented by etching of the third substrate in orderto clear a conductive membrane, and formation of a counter electrode.15. A method according to one of claims 10 to 13, the assembly beingimplemented by “Anodic bonding” or by sealing of the substrates.
 16. Amethod according to claim 15, the assembly being implemented by resinsealing.
 17. A method according to one of claims 11 to 13, the filterbeing a Fabry-Pérot interferential filter, its implementation bringinginto play: the implementation of a first mirror and a first associatedcontrol electrode, the implementation of a second mirror and a secondassociated control electrode, one out of the first and second mirrorsbeing fixed and the other being movable.
 18. A method according toeither of claim 17, the movable mirror being implemented by etching ofone of the substrates, the etched volume defining the micro-trough. 19.A method according to one of claims 11 to 13, the filter being aFabry-Pérot interferential filter, its implementation bringing intoplay: the implementation of a first mirror and a floating electrode, theimplementation of a second mirror and associated control electrodes, oneout of the first and second mirrors being fixed and the other beingmovable.
 20. A method according to one of claims 10 to 13, themicro-trough being implemented by etching of at least one semiconductorsubstrate until a membrane is obtained, the microphone next beingimplemented by formation of an electrode on the membrane and formationof electrodes on another substrate.
 21. A spectrometer according to oneof claims 1 to 4, the infrared source being able to be modulatedelectrically.
 22. A spectrometer according to claim 21, the infraredsource having a metallic grid or filament supported by a membrane abovea cavity etched in the first semiconductor substrate.
 23. A spectrometeraccording to one of claims 1 to 4, the infrared source having a metallicgrid or filament supported by a membrane above a cavity etched in thefirst semiconductor substrate.
 24. A spectrometer according to claim 23,the metallic grid or filament being made of silicon nitride, orplatinum, or tantalum, or titanium, or tungsten, or molybdenum, orchromium, or nickel, or one of their alloys.
 25. A spectrometeraccording to one of claims 2 to 4, the interferential filter being aFabry-Pérot tunable filter.
 26. A spectrometer according to claim 25,the interferential filter having a first, fixed, mirror and a second,movable, mirror, these mirrors delimiting, at rest, a resonant cavity oflength d, and first and second control electrodes respectivelyassociated with these first and second mirrors, the application of anelectrical voltage between the control electrodes allowingimplementation of a displacement of the movable mirror with respect tothe fixed mirror, and therefore modifying the length d of the resonantcavity.
 27. A spectrometer according to claim 26, the movable mirrorbeing implemented by a membrane situated above a cavity implemented inthe second semiconductor substrate.
 28. A spectrometer according toclaim 26, the movable mirror being composed of a membrane etched in thesecond semiconductor substrate.
 29. A spectrometer according to claim26, the mirrors of the filter being implemented by stacking ofmultilayers at λ/4, on the surface of the first and second or second andthird semiconductor substrates.
 30. A spectrometer according to claim26, the control electrodes forming an input diaphragm of themicro-trough.
 31. A spectrometer according to claim 26, the membrane ofthe tunable filter also constituting the movable mirror of the tunablefilter.
 32. A spectrometer according to claim 25, the Fabry-Pérottunable filter having: a first mirror, with which a floating electrodeis associated, a second mirror, with which a first and a second controlelectrode are associated, one out of the first and second mirrors beingprovided fixed and the other movable, a resonant cavity, of length d,delimited by the first and second mirrors, the application of anelectrical voltage between the two control electrodes bringing about adisplacement of he movable mirror with respect to the fixed mirror andtherefore modifying the length d of the resonant cavity.
 33. Aspectrometer according to claim 32, the control electrodes being formedon either side of a movable reflective area of the second mirror.
 34. Aspectrometer according to claim 33, the movable reflective area having acircular form.
 35. A spectrometer according to claim 32, the movablemirror being implemented by a membrane situated above a cavityimplemented in the second semiconductor substrate.
 36. A spectrometeraccording to claim 32, the movable mirror being composed of a membraneetched in the second semiconductor substrate.
 37. A spectrometeraccording to claim 32, the mirrors of the filter being implemented bystacking of multilayers at λ/4, on the surface of the first and secondor second and third semiconductor substrates.
 38. A spectrometeraccording to claim 32, the control electrodes forming an input diaphragmof the micro-trough.
 39. A spectrometer according to claim 32, themembrane of the tunable filter also constituting the movable mirror ofthe tunable filter.
 40. A spectrometer according to either of claims 1or 2, the microphone having a membrane and a first electrode associatedwith this membrane, both implemented on the third or the fourthsemiconductor substrate, and a second electrode, implemented on thefourth or the fifth semiconductor substrate.
 41. A spectrometeraccording to claim 3 or 4, the microphone membrane being implemented ina doped semiconductor material, the microphone also having a counterelectrode.
 42. A spectrometer according to claim 41, the microphonemembrane being situated on the surface of the third or the fourthsubstrate, the latter being etched under the membrane.